Hydrocarbons: Alkanes and Alkenes

This guide covers Topic 14, Hydrocarbons — subtopics 14.1 Alkanes and 14.2 Alkenes in full — from Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. This is AS Level content.

Before studying this

At IGCSE / O Level (subtopics 11.4 and 11.5), you learned that alkanes are saturated hydrocarbons that are generally unreactive except for combustion and substitution with chlorine, and that alkenes are unsaturated hydrocarbons made by cracking, distinguished from alkanes by decolourising aqueous bromine. That level stops at naming the reaction types — it does not require a mechanism.

AS Level adds three things IGCSE does not: the step-by-step free-radical substitution mechanism (initiation, propagation, termination) for alkanes, the step-by-step electrophilic addition mechanism for alkenes (including why Markovnikov’s rule holds), and a wider set of named reactions and reagents for producing each class of compound. You should also be comfortable with the vocabulary from Organic Mechanisms: An Introduction — homolytic fission, free radical, electrophile, curly arrows — before working through the mechanisms below.

Syllabus coverage

CAMBRIDGE INTERNATIONAL AS & A LEVEL CHEMISTRY 9701 — AS Level, Topic 14

14.1 Alkanes — the reactions by which alkanes are produced (hydrogenation of an alkene with H₂ and a Pt/Ni catalyst; cracking of a longer-chain alkane with heat and Al₂O₃); complete and incomplete combustion; free-radical substitution by Cl₂ or Br₂ in the presence of ultraviolet light, exemplified by ethane; the mechanism of free-radical substitution (initiation, propagation, termination); how cracking produces more useful alkanes and alkenes of lower Mr from heavier crude oil fractions; the general unreactivity of alkanes, including towards polar reagents, in terms of C–H bond strength and low polarity; the environmental consequences of carbon monoxide, oxides of nitrogen and unburnt hydrocarbons from combustion in the internal combustion engine, and their catalytic removal.

14.2 Alkenes — the reactions by which alkenes are produced (elimination of HX from a halogenoalkane using ethanolic NaOH and heat; dehydration of an alcohol using a heated catalyst such as Al₂O₃ or a concentrated acid such as concentrated H₂SO₄; cracking of a longer-chain alkane); electrophilic addition with hydrogen (H₂/Pt or Ni, heat), steam (H₂O(g)/H₃PO₄ catalyst), a hydrogen halide (HX(g), room temperature) and a halogen (X₂); oxidation by cold dilute acidified KMnO₄ to form a diol; oxidation by hot concentrated acidified KMnO₄, rupturing the C=C bond, and using the products to locate the position of an alkene linkage in a larger molecule; addition polymerisation, exemplified by ethene and propene; the use of aqueous bromine to test for a C=C bond; the mechanism of electrophilic addition, using bromine/ethene and hydrogen bromide/propene as examples; the inductive effect of alkyl groups on the stability of primary, secondary and tertiary carbocations formed during electrophilic addition, used to explain Markovnikov addition.

Alkanes: free-radical substitution

Alkanes are saturated — every carbon–carbon bond is a single covalent bond — which makes them generally unreactive: the C–H bonds are strong and only weakly polar, so alkanes resist attack by the polar and ionic reagents that attack other functional groups. Their two reactions worth knowing are combustion and substitution by chlorine or bromine under ultraviolet light.

Mechanism, exemplified by ethane and chlorine:

Initiation — UV light supplies enough energy to break the Cl–Cl bond homolytically:

Cl₂ → 2Cl• (each chlorine atom keeps one electron of the shared pair)

Propagation — two steps, each regenerating a radical so the chain continues:

Cl• + C₂H₆ → C₂H₅• + HCl

C₂H₅• + Cl₂ → C₂H₅Cl + Cl•

Termination — any two radicals combine, removing them from the chain:

Cl• + Cl• → Cl₂

C₂H₅• + C₂H₅• → C₄H₁₀

C₂H₅• + Cl• → C₂H₅Cl

Because chlorine keeps being regenerated in propagation, one initiation event can trigger many substitutions — this is why the reaction is described as a chain reaction. Further substitution beyond mono-substitution is possible in principle but only mono-substitution is required for AS Level.

Alkenes: electrophilic addition

The C=C double bond consists of a σ bond and a π bond. The π bond’s electron density lies above and below the plane of the molecule, exposed and accessible — this is what makes alkenes far more reactive than alkanes and what makes them susceptible to attack by electrophiles.

Mechanism, bromine and ethene: as the non-polar Br₂ molecule approaches the electron-rich π bond, the approaching electrons induce a temporary dipole in the bromine molecule (δ+/δ−). The π bond’s electrons then attack the δ+ bromine atom, forming a C–Br bond and breaking the Br–Br bond heterolytically, producing a bromide ion and a positively charged carbocation intermediate. The bromide ion then attacks the carbocation from the opposite face, completing the addition to give 1,2-dibromoethane.

Mechanism, hydrogen bromide and propene (Markovnikov addition): HBr is already polar (δ+ on H, δ− on Br). The π bond attacks the δ+ hydrogen, breaking the H–Br bond heterolytically and forming a carbocation on one of the two double-bond carbons. Two carbocations are possible — a secondary cation (on C2) or a primary cation (on C1) — and the reaction proceeds through whichever is more stable, because a more stable intermediate forms faster (lower activation energy).

Why the secondary cation is favoured: alkyl groups are electron-donating by the inductive effect, pushing electron density towards the positively charged carbon and stabilising it. A secondary carbocation has two alkyl groups donating into the charged carbon; a primary carbocation has only one. The secondary cation is therefore more stable, forms preferentially, and the bromide ion then attacks it to give the major product.

Worked example. Predict the major organic product of propene reacting with HBr, and explain the choice using inductive effects.

Propene is CH₃–CH=CH₂. Protonation of the double bond can place the positive charge on C2 (secondary carbocation, CH₃–CH⁺–CH₃) or on C1 (primary carbocation, CH₃–CH₂–CH₂⁺). The secondary carbocation is stabilised by electron donation from two alkyl groups (the methyl on one side and the CH₂⁺ fragment’s neighbouring carbon on the other), while the primary carbocation is stabilised by only one. The reaction proceeds via the more stable secondary carbocation, so bromide attacks C2. The major product is 2-bromopropane, CH₃–CHBr–CH₃, with 1-bromopropane forming only as a minor product.

Locating a double bond by oxidative cleavage

Hot concentrated acidified KMnO₄ breaks the C=C bond completely, and the identity of the fragments tells you where the double bond was: a terminal =CH₂ group is oxidised all the way to CO₂ and water, an internal =CH– group is oxidised to a carboxylic acid, and a fully substituted =CR₂ group is oxidised to a ketone. Working backwards from the products to the alkene’s structure is a standard structure-determination question.

Common mistakes

• Writing “Cl₂ → Cl⁺ + Cl⁻” for initiation. Homolytic fission under UV light splits the bond evenly into two radicals, each with one electron — not into ions.
• Forgetting that propagation is two separate steps. Each propagation step must both consume a radical and produce a new one, keeping the chain going; a single combined equation loses this detail and the marks that go with it.
• Predicting the minor product as the major one. Markovnikov addition follows the more stable carbocation, not simply “the carbon with more hydrogens” — reason from inductive stabilisation, not a memorised shortcut.
• Using cold dilute KMnO₄ conditions where hot concentrated conditions are needed, or vice versa. Cold dilute acidified KMnO₄ stops at the diol; only hot concentrated acidified KMnO₄ cleaves the C=C bond.

Quick revision checklist

• Alkane production: hydrogenation of an alkene, cracking of a longer alkane
• Free-radical substitution mechanism: initiation, propagation (two steps), termination, exemplified by ethane and chlorine
• Why alkanes are generally unreactive: strong, weakly polar C–H bonds
• Environmental impact of combustion products and catalytic removal
• Alkene production: elimination from a halogenoalkane, dehydration of an alcohol, cracking
• Electrophilic addition with H₂, steam, HX and X₂; oxidation with cold dilute vs hot concentrated acidified KMnO₄; addition polymerisation
• Electrophilic addition mechanism for bromine/ethene and HBr/propene
• Markovnikov addition explained by carbocation stability and inductive effects

Related resources

• Organic Mechanisms: An Introduction — the mechanism vocabulary and curly-arrow notation this resource assumes
• Halogenoalkanes: Nucleophilic Substitution and Elimination — halogenoalkanes made from alkanes and alkenes, and their own reactions
• Alcohols: Reactions and Oxidation — alcohols made from alkenes by electrophilic addition
• Addition Polymerisation — the polymerisation reaction of alkenes, covered in full
• Arenes and Halogenoarenes: Electrophilic Substitution and Reactivity — the A Level aromatic hydrocarbon, benzene
• Cambridge AS & A Level Chemistry hub

Written against Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Always check the current syllabus for your examination year.